AERIAL PHOTOGRAPHY AND FOREST MAPPING 517 



the region are available, as was the case in France, and the main need 

 for the photographic maps is to show the activities of the enemy. 

 Points on the photographs can be tied in with the corresponding points 

 on the scale maps, and the desired amount of detail filled in with almost 

 any degree of accuracy. The speed with which aerial mosaics can be 

 made is remarkable. The Division of Military Aeronautics recently 

 made a mosaic of the city of Washington and surrounding country 

 (see illustration on opposite page) in a total flying time of only 2%. 

 hours. The area covered was 27 square miles. 



The possibility of producing accurate topographic maps from photo- 

 graphs has been appreciated for many years, and the camera has been 

 used for topographic surveys to a limited extent in India, France, and 

 Italy, and almost exclusively in the Dominion of Canada. Cameras 

 used for this purpose are known as photo-theodolites, and are equipped 

 with suitable cross-hairs leveling devices, horizontal scale, and mag- 

 netic compass, as well as with a small telescope for the reading of verti- 

 cal angles. 



In mapping an area, suitable camera stations are selected and their 

 exact geographic location determined by means of primary or second- 

 ary triangulations or by other suitable method of survey, and over- 

 lapping views of the area are then taken from each camera station. 

 From the data obtained it is possible to construct on the drawing-boards 

 an accurate topographic map. 



It would likewise be possible to construct topographic maps from 

 aerial photographs, if certain necessary conditions were complied with. 

 These conditions are, briefly, as follows : 



1. The camera plate must be truly horizontal when the exposure is 

 made. 



2. The optical center of the negative must be determinate. 



3. The overlap between successive negatives must be great enough so 

 that the point on the ground lying in the optical axis of one negative 

 appears in both the preceding and succeeding negatives. 



4. The exact height of the camera above the ground at the instant of 

 exposure must be determinate. 



5. The whole survey must be tied in by base points of known geo- 

 graphical location. 



Several makes of aerial camera have been developed for this class of 

 work. They are arranged for flexible suspension in the airplane, and 

 means are provided for keeping them vertical during flight. These 

 means consist of g}Toscopic control or suitably cushioned pendulum or 

 plumb-bob action. 



